It has long been recognized by those skilled in the band saw blade art that a saw tooth edge having a "variable pitch" tooth pattern will avoid the problems of excessive noise and vibrations brought about by equally spaced teeth impacting the work in equal time intervals, with the end result being substantially reduced noise levels and increased blade life. Although band saw blades have generally developed as a distinct branch of the cutting tool art, separate from other cutting tools and other types of saws, it has also been common practice to use variable pitch tooth patterns on cutting edges of cylindrical hole saws for more reliable cutting operations.
Historically, "variable pitch" saw tooth patterns are composed of repetitive groups of saw teeth that are arranged with progressive variations in the pitch between successive pairs of teeth in the cutting direction of the saw blade. The variations are in pitch distances between corresponding points on adjacent teeth, e.g., the pitch distance between the tips of adjacent teeth of the saw. The variations in pitch start at the leading end of the saw and extend toward the trailing end of the saw and are generally cyclical from fine-to-coarse and back to fine again, or visa versa.
One skilled in the art of band saw blades will recognize references to "fine" and "coarse" in relation to a pattern of saw teeth, as referring to the pitch distance between one adjacent leading pair of teeth being less or fine in relation to a coarse, or greater pitch distance between another adjacent pair of trailing teeth in the pattern. It does not mean that such teeth are higher or lower than the others, as measured from a given reference line along the saw.
Examples of earlier attempts at variable pitch type saw blades are disclosed in U.S. Pat. No. 2,227,864 and U.S. Pat. No. 2,568,870. Both patents disclose saws having finer teeth at the leading end of the blade with the teeth gradually become coarser and thereafter again becoming fine. The cycle is repeated over the length of the saw. The gullets also go through the same progression. Thus, these patents relate to changing progressively both pitch and gullet sizes.
U.S. Pat. No. 4,179,967 (reissued as Re. 31,433 in 1983) discloses a variable pitch band saw blade, as shown in FIGS. 1-3, in which the gullet depth and pitch distance between adjacent teeth arranged along the saw first decrease progressively from a leading, unset raker tooth 11 that is the coarsest tooth, to the finest tooth 15, and then progressively increase again to a tooth having the same characteristics as the leading tooth. The teeth also have varying rake angles and the trailing teeth are alternately set laterally to the same extent. While this latter configuration of the '967 patent offers some advantages over the above-referenced '864 and '870 patents, particularly with respect to the reduction of noise and vibration, there remains a common tendency of reduced cutting rates and tooth breakage in all such variable pitch blades, especially when cutting large cross-sections of difficult-to-cut materials. It has been found that such reduced cutting rates and blade failures occur as a result of certain gullets that are too small to accommodate the size of chip material being cut by the blade particularly when being operated at the higher feed rates, as shown at C in FIG. 1.